Have you ever wondered what makes certain things glow under black lights?

For this experiment you will need:

• a black light
• petroleum jelly
• a piece of paper

First we’ll use the petroleum jelly as a kind of invisible ink. Dip your finger into the jelly, then use your finger to write a message on the piece of paper. Use more jelly if you need to – but this probably isn’t the time to write a long speech! When you’re finished, wipe any remaining jelly off your finger. Have the black light ready, then turn off the room lights and turn on the black light.

Can you see the message? Why is something that you couldn’t see in room light now visible when you can’t see any light?

First, let’s talk about the light. The reason black lights are called “black lights” is because they give off very little light that our eyes can see. Visible light contains a spectrum of colors ranging from red, through orange, yellow, green, and blue, to violet or purple. Beyond violet light in the spectrum is ultraviolet light, which our eyes cannot detect.

You may have heard of ultraviolet light if you know about sunburn. Sunburn is caused by a type of ultraviolet light, which scientists call “ultraviolet B” (UV-B). UV-B is higher in energy than the light from black lights, which is called “ultraviolet A” (UV-A). Black lights will not give you a sunburn.

If we can’t see ultraviolet light, why does the petroleum jelly glow under the black light?

Most of the time when we look at an object, we see light reflected from the surface of the object. But with a black light, there isn’t much visible light, so simple reflection of light doesn’t account for how bright the jelly glows. Petroleum jelly contains substances called phosphors. A phosphor absorbs radiation and emits it as visible light. So the phosphors in the jelly are absorbing the invisible ultraviolet radiation from the black light and emitting visible light.

Can you find anything else in your home that glows under black light?

One thing that usually glows brightly under black lights is a white shirt. Most laundry detergents contain “bluing agents” that are advertised as making the whites “whiter.” In fact, these agents are phosphors that respond to the UV-A radiation in normal light. The black light emphasizes their presence.

Another example of phosphors can be found on new $20 bills. As part of the government’s program to make currency harder to counterfeit, $20 bills issued since October, 2003, have a “security thread” that glows under ultraviolet light. The security thread is being introduced into $50 and $100 bills as well.

Glowing Hands

Can you think of a way to make your hands glow in the dark?

For this experiment you will need:

• a black light
• petroleum jelly
• latex gloves if you don’t want to get your hands messy (caution: some people are allergic to latex gloves!)
• someone to turn on the black light for you.

If you have Latex gloves, put them on your hands. Reach into the jar of petroleum jelly and scoop out enough jelly to cover both hands. Rub the jelly well over both hands, and then ask someone to turn off the lights in the room, and to turn on the black light. Hold your hand under the black light.

What do you see? Can you think of a way you could use this trick when telling ghost stories at night?

You will need

What to do

Cut a piece of yarn about 20 inches (40 cm) long.
Ask an adult to use the nail to carefully punch a hold in the center of the bottom of the cup.
Tie one end of the yarn to the middle of the paper clip.
Push the other end of the yarn through the hole in the cup and pull it through as shown in the picture.
Get a piece of paper towel about the size of a dollar bill, then fold it once and get it damp in the water.
Now it’s time to make some noise! Hold the cup firmly in one hand, and wrap the damp paper towel around the string near the cup. While you squeeze the string, pull down in short jerks so that the paper towel tightly slides along the string. If all goes well – you hear a chicken!

How does it work?

This is an example of how a sounding board works. The vibrations from the string would be almost silent without the cup, but when you add the cup, it spreads the vibrations and amplifies them (makes them louder.) Pianos and music boxes use wood to act as a sounding board to make the instrument louder.

MAKE IT AN EXPERIMENT

The project above is a DEMONSTRATION. To make it a true experiment, you can try to answer these questions:

1. What types of string or yarn makes the loudest sound? Which ones make the quietest?

2. Does the size of the cup affect the volume of the sound?

3. Try materials other than a paper towel to see if it affects the volume of the sound.

You will need

What to do

1. Turn on the faucet and slowly turn down the water until you have a VERY thin stream of water flowing.

2. Take the plastic comb and brush it through your hair ten times.

3. Now slowly bring the comb close the the flowing water, (without actually touching the water) If all goes well, the stream of water should bend towards the comb! Magic you ask? Not really.

How does it work?

When you brushed that comb through your hair, tiny parts of the atoms in your hair, called ELECTRONS, collected on the comb. These electrons have a NEGATIVE charge. Remember that, its important. Now that the comb has a negative charge, it is attracted to things that have a POSITIVE charge. It is similar to the way some magnets are attracted to certain metals.

When you bring the negatively charged comb near the faucet it is attracted to the POSITIVE force of the water. The attraction is strong enough to actually pull the water towards the comb as it is flowing! If you want to try another experiment with your comb, tear up pieces of tissue until they are as a small as you can get them…I mean really small! Then charge your comb again by brushing it through your hair, and bring it close to the tiny pieces of tissue. If the pieces are small enough they will jump off the table to the comb the same way that the water was pulled to the comb.It is all thanks to the wonders of static electricity.

MAKE IT AN EXPERIMENT

The project above is a DEMONSTRATION. To make it a true experiment, you can try to answer these questions:

1. Does water temperature affect how much the water bends?
2. Does the size of the comb affect the static power?
3. Does the amount of moisture in that air affect the static power? Try it after someone has taken a shower in the room.
4. Does the material that the comb is made of affect the static power?

If you’ve ever wondered why Diet Coke and Mentos react so strongly to one another, well, wonder no more.

To start, it should be noted that it’s not just Diet Coke and Mentos that “react”; other carbonated beverages will also readily respond to the addition of Mentos. What’s going on here is that Mentos has thousands of small pores on its surface disrupting the polar attractions between the water molecules, creating thousands of ideal nucleation sites for the gas molecules in the drink to congregate. In non-sciency terms, basically, this porous surface creates a lot of bubble growth sites, allowing the carbon dioxide bubbles to rapidly form on the surface of the Mentos. If you use a smooth surfaced Mentos, you won’t get nearly the reaction.

The buoyancy of the bubbles and their growth in size will quickly cause the bubbles to leave the nucleation site and rise to the surface of the soda. Bubbles will continue to form on the porous surface and the process will repeat, creating a nice foamy result.

In addition to that, the gum arabic / gelatin ingredients of the Mentos, combined with the potassium benzoate, sugar or (potentially) aspartame, in Diet sodas, also help in this process. In these cases, the ingredients end up lowering the surface tension of the liquid, allowing for even more rapid bubble growth on the porous surface of the Mentos: higher surface tension = more difficult environment for bubbles to form. (For your reference, compounds like gum arabic that lower surface tension are called “surfactants”).

As to why diet sodas like Diet Coke produce such a bigger reaction, it’s because aspartame lowers the surface tension of the liquid much more than sugar or corn syrup will. You can also increase the effect by adding more surfactants to the soda before you add the Mentos, like adding a mixture of dishwasher soap and water.

Another factor contributing to the size of the geyser is how rapidly the object causing the foaming sinks in the soda. The faster it sinks, the faster the reaction can happen, and faster reaction = bigger geyser; slower reaction may release the same amount of foam overall, but also a much smaller geyser. This is another reason Mentos works so much better than other similar confectioneries. Mentos are fairly dense objects and so tend to sink rapidly in the liquid. If you crush the Mentos, so it doesn’t sink much at all, you won’t get nearly the dramatic reaction.

Yet another factor that can affect the size of the Mentos / Coke geyser is the temperature of the soda itself. The higher the temperature, the bigger the geyser due to gases being less soluble in liquids with a higher temperature. So, basically, they are more “ready” to escape the liquid, resulting in a faster reaction.

Note that while caffeine is often cited as something that will increase the explosive reaction with the soda, this is not actually the case, at least not given the relatively small amount of caffeine found in a typical 2-liter bottle of soda generally used for these sorts of Diet Coke and Mentos reactions. If you add enough caffeine, you will see a difference, but the levels required here to see a significant difference are on the order of the amount that would kill you if you actually consumed the beverage. (See: How Much Caffeine Would It Take to Kill You)

You’ll also sometimes read that the acidity of the soda is a major factor in the resulting geyser. This is not the case either. In fact, the level of acidity in the Coke before and after the Mentos geyser does not change, negating the possibility of an acid-based reaction (though you can make such an acid based reaction using baking soda).

What You Need:

What You Do:

1. Roll up the sheet of paper or cardstock the long way to form a tube that is about the diameter of the lens with the shortest focal length. This will be the eyepiece. Tape the edges of the eyepiece lens to one end of the tube as neatly as possible.

2. Tape the second lens neatly to the end of the paper towel tube. Insert the empty end of the paper tube into the cardboard tube. Now your telescope is ready to be used!

3. Look through the eyepiece and point the other end of your telescope at a distant object. Slide the two tubes in and out until the object comes into focus. You will see the image upside down and magnified. If you have trouble focusing the telescope, you may need to lengthen the tube, either by using a larger piece of paper for the eyepiece end or a longer cardboard tube (such as from a wrapping paper roll).

What Happened:

You have just built a simplified version of a refractor telescope. The lenses at each end work together to focus on a distant object and magnify it so that your eye can see it better. The lens on the outside tube is called the objective lens. This lens collects light from whatever you point the telescope at. The lens at the other end of the telescope is called the eyepiece lens. It takes the light that the objective lens has collected and makes it bigger so that it takes up more space on the part of your eye that allows you to see, so that when you see the image that your telescope is focused on, you see it several times larger than you can see it with your eye alone.